STIMULATION OF CA2-ACTIVATED CA2+ CHANNELS AS A POTENTIAL MECHANISM INVOLVED IN NON-GENOMIC 1,25(OH)(2)-VITAMIN D-3-INDUCED CA2+ ENTRY IN SKELETAL-MUSCLE CELLS( RELEASE)

As in other vitamin D target cells, activation of voltage-dependent Ca 2+ channels (VDCC) mediates the fast, non-genomic, 1,25(OH)(2)D-3 stim ulation of Ca2+ influx in skeletal muscle cells (SMC). 1,25(OH)(2)D-3 has also been shown to rapidly induce the release of Ins(1,4,5)P-3 in SMC. Experiments were performed to investigate whether Ca2+ in the mec hanism by which 1,25(OH)(2)D-3 regulates Ca2+ entry into these cells. In cultured chick SMC loaded with Fura-2/AM the hormone (10(-12)-10(-8 ) M) induced a rapid (30 sec) followed by a sustained (up to 5 min) in crease in intracellular Ca2+ concentration ([Ca2+](i)) associated to C a2+ mobilization from internal stores and influx of extracellular Ca2, respectively. Thus, the initial, transient, 1,25(OH)(2)D-3-dependent increment in [Ca2+](i) could be observed in Ca2+-free medium and was abolished by the PLC inhibitor U73122. Readdition of Ca2+ to cells tha t had undergone the initial 1,25(OH)(2)D-3-induced [Ca2+](i) rise in C a2+ free medium resulted in a fast increment in [Ca2+](i) indicating t he existence of a hormone-activated CRAC entry pathway. The sustained phase of the Ca2+ response to 1,25(OH)(2)D-3 was only partially (60%) suppressed by nifedipine, whereas lanthanum (10 mu M) completely aboli shed the hormone effects. Accordingly, depletion of intracellular Ca2 stores by thapsigargin re-produced 1,25(OH)(2)D-3-induced Ca2+ influx , inhibiting any further response to the sterol. 1,25(OH)(2)D-3 increa sed the rate of quenching of Fura-2 fluorescence by Mn2+, indicating a ctivation of Mn2+ permeable channels. Altogether, these results provid e the first evidence involving CRAC channels in the rapid modulation o f Ca2+ entry in animal cells by 1,25(OH)(2)D-3. (C) 1997 Academic Pres s.